Oil (and maybe also do a conditioner coat) before braiding in the morning, should help protect her hair. The braids will probably be a bit messy but should be able to survive the swim cap. If they don't then someone should be able to help her undo them and at least put it into a ponytail. If they let them rinse before/after tell her to make sure to wet her hair down. Use a clarifying shampoo on days she swims.
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